Typically, a tungsten halogen lamp comprises a tubular glass bulb, a molybdenum lead-in foil, a tungsten coil filament and halogen gas filled in the space of the bulb together with inert gas, such as argon. The molybdenum leading foil is embedded in a sealed end of the bulb. The tungsten coil filament is housed in the space of the bulb and electrically connected to the molybdenum lead-in foil.
Recently, tungsten halogen lamps have been coated with a prescribed optical film which passes visible light therethrough, but reflects infrared rays. The optical film is coated on at least one of the inner and outer wall surfaces of the bulb.
In such a tungsten halogen lamp, the visible light emitted from the tungsten filament passes through the optical film and radiates to the outside of the bulb. The infrared rays are reflected by the optical film. A part of the reflected infrared rays return to the filament and heat it up. Thus, the luminous efficiency of the tungsten filament is increased. Furthermore, the optical film reduces the component of infrared rays contained in the light radiated outside the lamp, so that the optical film decreases damage to objects being illuminated caused by excessive heat.
Another portion of the reflected infrared rays is applied to another portion of the optical film and again is reflected thereby toward the inside of the bulb. Thus, a relatively large amount of the infrared rays reach the sealed part and heat the molybdenum lead-in foil embedded in the sealed part.
The temperature of the bulb wall in such a tungsten halogen lamp is set to a relatively high temperature in order to carry out a halogen regeneration cycle on the wall surface of the bulb. The high temperature of the bulb wall is transmitted by conduction to the sealed part of the bulb.
Thus, the molybdenum lead-in foil is intensively heated by the infrared rays directly and indirectly applied thereto. The molybdenum lead-in foil deteriorates due to oxidization caused by the high temperature. This deterioration damages the sealing of the sealed end around the molybdenum lead-in foil, so that gas leakage from the bulb is accelerated. As a result, the life of the halogen lamp is decreased.